Introduction

“Who are you?” Authentication is the process where an entity proves the identity of another entity, typically through credentials, such as a username and password.

Depending on your requirements, there are several available authentication mechanisms to choose from. If they are not correctly chosen and implemented, the authentication mechanism can expose vulnerabilities that attackers can exploit to gain access to your system.

The storage of passwords and user credentials is also an issue from a defense in depth approach, but also from a compliance standpoint. The following section also discusses password storage and what to review for.

The following discusses aspects of source code relating to weak authentication functionality. This could be due to flawed implementation or broken business logic: Authentication is a key line of defence in protecting non-public data, sensitive functionality.

Weak Passwords and Password Functionality

Password strength should be enforced upon a user setting/selecting ones password. Passwords should be complex in composition. Such checks should be done on the backend/server side of the application upon an attempt to submit a new password.

Use the following recommendations to improve Forms authentication security:

Partition your Web site.

Set protection=“All”.

Use small cookie time-out values.

Consider using a fixed expiration period.

Use SSL with Forms authentication.

If you do not use SSL, set slidingExpiration = “false”.

Do not use the <credentials> element on production servers.

Configure the <machineKey> element.

Use unique cookie names and paths.

For classic ASP pages, authentication is usually performed manually by including the user information in session variables after validation against a DB, so you can look for something like:

Session ("UserId") = UserName
Session ("Roles") = UserRoles

Cookieless Forms authentication

Authentication tickets in forms are by default stored in cookies (Authentication tickets are used to remember if the user has authenticated to the system), such as a unique id in the cookie of the HTTP header. Other methods to preserve authentication in the stateless HTTP protocol. The directive cookieless can define thet type of authentication ticket to be used.

Types of cookieless values on the <forms> element:

UseCookies – specifies that cookie tickets will always be used.

UseUri – indicates that cookie tickets will never be used.

AutoDetect – cookie tickets are not used if device does not support such; if the device profile supports cookies, a probing function is used to determine if cookies are enabled.

UseDeviceProfile – the default setting if not defined; uses cookie-based authentication tickets only if the device profile supports cookies. A probing function is not used.

cookieless="UseUri" : What may be found in the <forms> element above

When we talk about probing we are refering to the user agent directive in the HTTP header. This can inform ASP.NET is cookies are supported.

Password Storage Strategy

The storage of passwords is also of concern, as unauthorized access to an application may give rise to an attacker to access the area where passwords are stored.

Passwords should be stored using a one-way hash algorithm. One way functions (SHA-256 SHA-1 MD5, ..;) are also known as Hashing functions. Once passwords are persisted, there is not reason why they should be human-readable. The functionality for authentication performs a hash of the password passed by the user and compares it to the stored hash. If the passwords are identical, the hashes are equal.

Storing a hash of a password, which can not be reversed, makes it more difficult to recover the plain text passwords. It also ensures that administration staff for an application does not have access to other users’ passwords, and hence helps mitigate the internal threat vector.

Salting:
Storing simply hashed passwords has its issues, such as the possibility to identify two identical passwords (identical hashes) and also the birthday attack. A countermeasure for such issues is to introduce a salt. A salt is a random number of a fixed length. This salt must be different for each stored entry. It must be stored as clear text next to the hashed password:

Bad/weak error handling can be used to establish the internal workings of the authentication functionality such as giving insight into the database structure, insight into valid and invalid user IDs, etc.